Method of and material for coating metal surfaces



ployed but with various difficulties.

Patented June 23, 1936 UNITED STATES PATENT. OFFICE METHOD OF AND MATERIAL FOR COATING METAL, SURFACES No Drawing. Application June 4, 1934, Serial No. 729,030

15 Claims. '(Cl. 148-6) v in which the chemical deposition is accelerated by an oxidizing agent, particularly where such oxidizing agent is a nitrate.

The object of the invention is to provide a material which can be premixed, and transported and stored without serious deterioration, and a method by which this material may be'used to successfully coat metal surfaces.

While the broader features of the invention are applicable to quite a variety of metal coating operations, for convenience one specific operation will be described first and the application of this invention thereto, and then other applications of the invention to other specific metal coating processes will be indicated.

It has been common practice to provide iron or steel surfaces with phosphate coatings for rust-proofing and paint base purposes, and in recentyears it has been found that this coating with phosphates is accelerated by nitrates in the solution. One solution in quite wide use for coating iron or steel surfaces with phosphates comprises an aqueous solution of manganese dihydrogen phosphate with a nitrate as an accelerating'agent. Sodium nitrate has been emplayed for this purpose, but the sodium gradually builds up in the solution and may cause some difficulties in proper replenishment of the solution. Various other nitrates have been em- Manganese nitrate has been found the most satisfactory in many respects, but this nitrate melts at such a low point that it is not by itself a material which can be readily stored and transported as a powder and, if mixed with the manganese dihydrogen phosphate, reactions take place within the powder which seriously decrease the effectiveness thereof.

By the present invention barium nitrate is employed in place of manganese nitrate, and a substantially molecular equivalent amount of manganous sulphate is mixed therewith. Both of these powders remain dry during ordinary atmospheric conditions and can be mixed with manganese dihydrogen phosphate without serious deterioration in ordinary commercial transportation and storing. Accordingly, a mixture of ployed to form or replenish a phosphate coating solution as desired.

When dissolved, the barium nitrate and manganese sulphate react in the solution to form manganese nitrate and barium sulphate. The 5 barium sulphate being substantially insoluble is promptly precipitated out of the solution. In this manner the desired solution of manganese phosphate and manganese nitrate is secured without the inconvenience of shipping and storl0 ing manganese nitrate as such. In this way a much better material to handle is obtained and the necessary ingredients for forming or replenishing the solution can be mixed as one powder, so that the solution can be more readily formed 15 or replenished by merely adding the required amounts of the premixed material.

For certain purposes a small amount of a soluble copper salt is also desirable in the solution, as this aids in the prompt and uniform initia- 20 tion of the coating operation. Copper nitrate would be entirely satisfactory for this purpose so far as the actual coating operation is concerned, but copper nitrate,like manganese nitrate, does not keep well and, therefore, intro- 25 duces serious difliculties in the premixing of the material or the storing and transportation thereof. Copper sulphate can be used quite satisfactorily for this purpose. A certain amount of the acid radical S04 can be tolerated in the phos- .30 phate solution, but any considerable amount thereof interferes with a proper coating operation. For this reason the introduction of copper sulphate or any other sulphate into a bath which is to be continuously replenished for long 35 periods of time results in undesirable building up of sulphate in the solution. Also certain amounts of sulphates are introduced into the solution by the available water supply in many localities. Where the hot process is used and the solution 40 is continuously kept at or near the boiling point, the evaporation of water and the replenishment thereof gradually builds up appreciable amounts of sulphates in the solution. Likewise, there are cases wherev it is desirable to coat iron'or steel 45 surfaces which have previously been pickled in sulphuric acid solutions and where the sulphuric acid has-not been completely rinsed off. In such cases the radical S04 is carried into the solution by the work treated therein.

The introduction of barium nitrate takes care of the S04 radical, however introduced into the solution, as the substantially insoluble barium sulphate isprecipitated and settles out of the solution without detriment.

In the solution of the kind described it is preferable to introduce the barium nitrate in approximately the proper amount to unite with the S04 radical and thereby remove both the barium and the S04 from the solution in approximately the proportions in which they are introduced therein. However, if there is a slight excess of barium, this enters into the coating, apparently as barium phosphate, and in slight amounts is not any detriment. Therefore, the barium may be introduced in amountssufficient to certainly unite with all S04 introduced into the solution in any way and a slight excess of the barium is taken care of automatically by the deposition of the barium in the coating as barium phosphate. This excess of barium, however, should not be great as only a small amount of barium can remain in the solution without noticeable deterioration in the resulting coating operation.

In accordance with these principles there can be a considerable variation in the proportions of the different ingredients, but one proportion of the materials which has been found quite satisfactory for continued replenishment of a phosphate coating bath for iron or steel is'made up a as follows:

Manganese dihydrogen phosphate pounds 186 Barium nitrate..--- do 42 Manganese sulphate (MnSOnl/ZI-IzO) pounds 22 Copper sulphate or carbonate ounces 6 There are numerous other nitrates which may be employed to accelerate the solution. The nitrates of sodium, magnesium, etc., leave a relatively inert residue in the solution, while the metal of other nitrates is taken out of the solution by the coating process. Of the latter, the nitrates of zinc'and cadmium are preferable for coating purposes. Zinc nitrate and cadmium nitrate may be handled as dry powder under favorable conditions, but do not have the resistance to atmospheric conditions which is afis not as marked as where manganese nitrate is or steel formed by a bath relatively high in manganese content is more rust resistant than where the iron content of the bath is relatively high. Nitrate of iron is too readily afiected to resist exposure in trying weather conditions of heat and humidity and, therefore, where ferrous nitrate is to be employed in the bath it is advantageous to introduce it by adding barium nitrate and ferrous sulphate rather than by attempting to use the ferrous nitrate directly.

aoeaeao To summarize this discussion of the use of barium nitrate in a phosphate solution for coating iron orsteel, it may be pointed out that it is very advantageous to use this material in at least equal molecular proportions to the copper nitrate and manganese nitrate desired in the solution, the copper and manganese being introduced in the form of sulphates. Where it is desired to have zinc nitrate in the solution, it is desirable to introduce it by adding barium nitrate and zinc sulphate, although not as necessary as in the case of manganese, because zinc nitrate is more resistant to atmospheric conditions than is manganese nitrate. Where any nitrate or iron, cadmium, or other metal that will enter into the coating, is to be introduced, this still can be done readily by the use of barium nitrate and the sulphates of the respective metals.

It is also known that zinc can be coated in a phosphate solution in a manner similar to the coating of iron. However, in a zinc coating solution, manganese is not desirable and iron is a very good accelerating agent. For this reason in preparing material for coating the zinc, it is preferable to use ferrous sulphate rather than manganese sulphate in connection with barium nitrate for accelerating purposes. Zinc and/or ferrous dihydrogen phosphate may be used instead of manganese phosphate for coating zinc.

Various other metals below zinc in the electro-' motive series have an accelerating eifect upon the coating of zinc and may be introduced as sulphates if desired, the importance of introducing them as sulphates together with equivalent amounts of barium nitrate being dependent upon the ease with which the nitrate of the respective metal may be handled in powder form.

In coating zinc, however, the barium should not be in excess of the S04 radical as it is not automatically taken care of satisfactorily in the coating operation where it is in excess even in small amounts, as is the case in coating iron.

The other heavy alkaline earth metals, calcium and'strontium, act somewhat similarly to barium.

Calcium sulphate is soluble only to a small extent, but is more soluble thanbarium sulphate, and in a solution for coating iron or steel calcium nitrate is not as satisfactory as barium nitrate for removal of the S04, and must be used with more care. In a solution for coating zinc, calcium nitrate can be used in place of barium nitrate, in combination with ferrous sulphate or the sulphates of other accelerating metals, with less difficulty than is involved in using the calcium nitrate in a solution for coating iron.

Strontium may be present in considerable quantities in a phosphate solution without noticeable injurious effect upon its coating action on iron, and strontium may be readily precipitated from such a solution as sulphate. While there are some differences between the effects of strontium and barium, strontium nitrate is -the nearest equivalent for barium nitrate known to applicants, for the purposes for which barium nitrate is recommended in this specification. Barium nitrate is preferred mainly because it is cheaper.

Metal surfaces can be coated also with solutions of various organic acids in a manner somewhat similar to the phosphate coating process mentioned above. The organic acids which operate for forming coatings of salts thereof on metal surfaces in a manner somewhat analogous to the action of phosphate solutions are the dicarboxylic and hydroxydicarboxylic acids of the aliphatic series, the carboxylic acids, having but one carboxyl group, and the sulphonic acids of the arcmatic series, and phosphonic acids. For coating iron the combination of barium nitrate and manganese sulphate is very successful as an accelerator with these acids. For coating zinc with these acids, barium nitrate and ferric sulphate operate very successfully. With oxalic acid and the like, ferric sulphate is preferable to ferrous sulphate because ferric oxalate is soluble while ferrous oxalate is insoluble, so that the ferrous ion tends to precipitate out of the solution. For similar reasons calcium nitrate cannot be employed in these solutions in place of'barium nitrate, because the calcium is promptly precipitated as calcium oxalate and, therefore, does not eliminate the S04 radical.

When accelerated with copper or other accelerating metal and an oxidizing agent, the organic acids mentioned, or many of them, can be employed successfully to coat lead, tin, copper and their alloys. For this purpose copper sulphate with an equivalent amount of barium nitrate may be used.

There have been numerous solutions where various soluble salts of metals have been introduced in chemical solutions from which they were precipitated either as metallic coating or a coating of oxide on suitable metal surfaces. For example, a solution containing salts of nickel or cobalt with a soluble salt of copper in proper proportion will form a firmly adherent coating upon either zinc or iron. This operation is accelerated by the use of a nitrate in the solution. Accordingly this solution may be advantageously formed by the use of barium nitrate and the sulphates of the respective metals.

As will be seen from the foregoing, there are many different solutions. of chemicals which form coatings on the surfaces of various metals which coatings comprise the salts of various metals or the metals themselves chemically formed or freed in situ on the surface of the metal to be coated. In substantially all instances this operation seems to be improved and accelerated by the use of an oxidizing agent such as a nitrate in the solution. Wherever the metal, which is to form a part of the coating, cannot be advantageously'introducedas a nitrate, it may be introduced as a sulphate with the addition of a chemically equivalent amount of barium nitrate unless there is some positively contrary indication. For example, in the solution last mentioned where salts of cobalt or nickel are used with a salt of copper to coat zinc or iron, the solution is preferably acidulated and sulphuric acid is a very good material to use for such acidulation. In that case, of course, barium nitrate could not be used with economy as it would be promptly precipitated by the sulphuric acid in the solution and in turn would weaken the acidulation of the solution. However,

such a. solution may beacidulated by many other acids where such an effect would not result.

While any soluble dihydrogen phosphate can be used in a coating solution provided it hydrolizes to for'mfree phosphoric acid, or acid is provided in any way to acidulate the solution, for coating zinc and iron surfaces the dihydrogen phosphates of the metals from manganese to iron, inclusive, in the electromotive series, that is manganese, zinc, cadmium and iron, are the best phosphates to use commercially.

As indicated above various accelerating metals may be used in different coating compositions and enter into the coating. For use in accord.-

ance with the method claimed herein, any soluble sulphate of a metal as little basic as manganese and the nitrate of which also is soluble can be used in some coating solution in accordance with the principles indicated above andmay be introduced with the nitrate of a heavy alkaline earth metal in order to produce the accelerating nitrate in the solution.

It will be understood that in the claims where heavy alkaline earth metal is referred to, it is intended to cover calcium, strontium and barium, as the sulphate of magnesium is not insoluble and, therefore, it is not desired to include magnesium in the group, as has been sometimes done where reference was made simply to alkaline earth metals.

What we claim is:-

l. The method expediting a metal-coating solution which forms on a metal surface, by chemi cal reaction with the surface, a coating contain- ,series, carboxylic acids, having but one carboxyl group, and sulphonic acids of the aromatic series,

and phosphonic acids, where the coating action of the solution is expedited by NO: in the solution, the process which consists in introducing into the solution a soluble sulphate of a metal that enters into the coating produced by the solution and an approximately chemically equivalent amount of a nitrate of a heavy alkaline earth metal, that will form in the solution an insoluble sulphate and substantially freeing the solution from S04 and the heavy alkaline earth metal by precipitation thereof as sulphate.

3. A method of coating a metal of the group consisting of zinc, iron and their alloys, which consists in dissolving in water dihydrogen phosphate of a metal of the group consisting of manganese, zinc and iron, sulphate of a metal of the group consisting of manganese, zinc and iron, and nitrate of a metal of the group consisting of barium,'strontium and calcium, the sulphate being in nearly chemically equivalent proportion to the nitrate and applying said solution to the surface to be coated.

4. A method of coating iron, which consists in successively immersing a series of iron-surfaced articles in a dilute bath containing manganese dihydrogen phosphate, and replenishing the bath with manganese dihydrogen phosphate, manganese sulphate and barium nitrate, the sulphate and nitrate being in approximately equimolecular proportions.

5. A method of coating iron, which consists in dissolving in a solution of oxalic acid manganese sulphate and nitrate of heavy alkaline earth metal, precipitating as sulphate the major portion of the S04 and heavy alkaline earth metal, and applying the solution to the iron surface.-

6. A method of coating zinc, which consists in dissolving in a solution of oxalic acid ferric sulphate and nitrate of barium or strontium,

\ precipitating as sulphate the major portion of the S04 and barium or strontium, and applying the solution to the zinc surface.

7. A composition of matter for use in a metal coating solution comprising an acid powder of the group consisting of dihydrogen phosphates, dicarboxylic and hydroxydicarboxylic acids of the aliphatic series, carboxylic acids, having but one carboxyl group, and sulphonic acids of the aromatic series, and phosphonic acids, mixed with a soluble sulphate of a metal as little basic as manganese and the nitrate of which is soluble, together with nitrate of a heavy alkaline earth metal, the sulphate and nitrate being in approximately chemically equivalent amounts.

8. A composition of matter for use in a solution for coating metal of the group consisting of iron, zinc, and their alloys, comprising dihydrogen phosphate of metal in the group consisting of manganese, zinc, cadmium and iron, sulphate of metal in the last said group, and nitrate of a heavy alkaline earth metal, the sulphate being in nearly chemically equivalent proportion to the nitrate.

9. A composition of matter for use in a solution for coating, iron, comprising manganese dihydrogen phosphate, manganese sulphate and nitrate of barium or strontium, the sulphate and nitrate being in approximately equimolecular proportions.

10. A composition in accordance with claim 14, and in which the nitrate is of barium and in an amount slightly greater than chemically equivalent to the amount or sulphata.

11. A composition of matter for use in a, so1u-- tion for coating zinc, comprising dihydrogen phosphate of metal of the group consisting of,

zinc and iron, ferrous sulphate, and nitrate of a heavy alkaline earth metal, the sulphate and nitrate being in approximately equimolecular proportions.

12. A composition of matter for forming a so lution for coating iron, comprising oxalic acid,

manganese sulphate, and nitrate of barium or strontium, the sulphate and nitrate being in approximately equimolecular proportions.

13. A composition of matter for use in a solution for coating zinc, comprising oxalic acid, ferric sulphate and nitrate of barium or strontium, the sulphate and nitrate being in approximately equimolecular proportions.

14. A composition of matter for use in a solution for coating iron, comprising manganese sulphate half hydrate, and barium nitrate in approximately the proportions of 22 lbs. of the sulphate to 42 lbs. of the nitrate.

15. A composition of matter for use in a solution for coating zinc, comprising sulphate of iron and nitrate of barium, the said sulphate and nitrate containing amounts of metal substantially equivalent chemically.

ROBERT R. TARNER. HERMAN J. LODEESEN.

CERTIFICATE CORRECTION.

Patent No. 2,045,499. June, 25, 1936.

ROBERT R. TANNER, ET AL.

It is hereby certified that error appears in the. printed specification pf the above numbered patent requiring correction as follows: Page 4,' first column, line 31, claim 10, for the claim referenee numeral "14" read 9; and that the said Letters Patent should be read with this correctqn therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day of September, A. D. 1936.

Leslie Frazer (Seal) Acting. Commissioner of Patents 

